Set 'first' flag to print holes properly

Author: rocallahan

src/DumpCommand.cc

@@ -340,6 +340,7 @@ static void dump_events_matching(TraceReader& trace, const DumpFlags& flags,
               if (!first) {
                 fputs(", ", out);
               }
+              first = false;
               fprintf(out, "%p-%p", (void*)h.offset, (void*)(h.offset + h.size));
             }
             fputs("]", out);